Silicon-based solar cells, which are currently the dominant solar cell type, exhibit high photoelectric conversion efficiency, but incur significant environmental impact in terms of resources, carbon dioxide emissions during production, and cost. Therefore, research is being conducted, on a global scale, on solar cells that are clean, and have fewer resource constraints, in the form of photoelectric conversion elements that use organic semiconductor materials, in particular on so-called organic thin-film solar cells having a structure such that an organic semiconductor is sandwiched between electrodes that have dissimilar work functions.
In 2005, Heeger et al. produced an ITO/PEDOT:PSS/P3HT:PCBM blend film/Al-type cell (hereafter also referred to as “normal-type solar cell”), which attained a photoelectric conversion efficiency of 5% (Non-Patent Document 1).
Herein, “ITO” denotes indium tin oxide; and “PEDOT:PSS” denotes a polymer compound comprising poly-3,4-ethylenedioxythiophene (PEDOT) and polystyrene sulfonate (PSS), having the structure below.

Further, “P3HT:PCBM blend film” denotes a filmy body comprising a mixture of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), wherein the constituent substances have the respective structures below.

The highest value of photoelectric conversion efficiency of organic thin-film solar cells currently reported is of 6.5% for a tandem-type cell, produced by Heeger et al. in 2007, using a P3HT:PC70BM blend film and a PCPDTBT:PCBM blend film (Non-Patent Document 2).
Efficient transportation, to the respective electrodes, of the positive and negative carriers that are generated in the organic semiconductor as a result of light irradiation is a particularly important issue in organic thin-film solar cells in terms of enhancing the performance of the organic thin-film solar cell as a photoelectric conversion element.
In the above-described normal-type solar cell it is expected that hole-transport efficiency is enhanced by a hole-transport layer that comprises PEDOT:PSS.
The inventors have proposed techniques wherein an oxide semiconductor functioning as a hole-blocking layer is interposed between an anode electrode and an organic semiconductor layer from the viewpoint of enhancing electron transport efficiency (for instance, Patent Documents 1 and 2). As disclosed in Patent Document 2, energy conversion efficiency (η) is enhanced by interposing an oxide semiconductor layer (hole-blocking layer), comprising amorphous titanium oxide, between an ITO electrode and an organic semiconductor layer comprising a P3HT:PCBM blend film.
Patent Document 1: JP 2004-319705 A
Patent Document 2: JP 2009-146981 A
Non-Patent Document 1: Wanli Ma, Cuiying Yang, Xiong Gong, Kwaghee Lee, Alan J. Heeger, Adv. Funct. Mater. 15, 1617-1622 (2005)
Non-Patent Document 2: Jin Young Kim, Kwanhee Lee, Nelson E. Coates, Daniel Moses, Thuc-Quyen Nguyen, Mark Dante, Alan J. Heeger, Science, 317, 222-225 (2007)